1. Field of the Invention
The present invention relates to surface-acoustic-wave-sensor-included oscillator circuits including surface acoustic wave sensors, which are used as resonators, for use in, for example, biosensors or gas sensors. More particularly, the present invention relates to a surface-acoustic-wave-sensor-included oscillator circuits using a surface acoustic wave sensor that detects a target substance on the basis of a variation in frequency due to mass loading and to a biosensor apparatus using the surface-acoustic-wave-sensor-included oscillator circuit.
2. Description of the Related Art
Various surface acoustic wave sensors using surface acoustic wave devices have been proposed in order to detect various substances. For example, surface acoustic wave sensors for detecting biological materials, such as deoxyribonucleic acids (DNAs) or antibodies, have reaction films that react only to specific biological materials, such as DNAs or antibodies, on their surface acoustic wave devices. In such surface acoustic wave sensors, the DNAs or antibodies respond to the reaction films and are bound to the reaction films to load the mass on the surface acoustic wave devices. The presence or concentration of the DNAs or antibodies is detected on the basis of a variation in frequency due to the mass loading.
Japanese Unexamined Patent Application Publication No. 10-90270 (Patent Document 1) discloses an example of a surface acoustic wave sensor of this type. The surface acoustic wave sensor described in the related art is capable of detecting 2-methylisoborneol (2-MIB), which is a musty odor element contained in water. As shown in FIG. 11, a surface acoustic wave sensor 101 has interdigital electrodes 103 and 104 and a metal thin film 105 formed on a piezoelectric substrate 102. Amplifiers 106 and 107 are connected between the interdigital electrode 103 and the interdigital electrode 104. A mixer 108 is connected downstream of the interdigital electrode 104 at the output side and the amplifiers 106 and 107. The surface acoustic wave sensor 101 is structured such that an output is provided through the mixer 108.
In the surface acoustic wave sensor 101 described in the related art, an OVA-camphor complex is immobilized over the piezoelectric substrate 102. This OVA-camphor complex functions as a reaction film, and the 2-MIB is detected on the basis of a reaction of the 2-MIB to the OVA-camphor complex.
Specifically, the camphor-protein complex antigen, which has a structure similar to that of the 2-MIB being the musty odor element, is immobilized in the surface acoustic wave sensor 101. The surface acoustic wave sensor 101 is dipped in a solution to be measured, which contains an anti-2-MIB-antibody of a certain concentration specifically bound to the 2-MIB, and the 2-MIB whose concentration is unknown, in the solution, competitively reacts to the camphor-protein complex antigen. The amount of anti-2-MIB-angibody bound to the camphor-protein complex antigen immobilized over the surface acoustic wave sensor 101 is yielded on the basis of a variation in output level due to the mass loading on the surface acoustic wave sensor. A difference between the amount of anti-2-MIB-antigen bound to the camphor-protein complex antigen and the amount of the bound antibody when the 2-MIB does not exist is calculated to determine the concentration of the 2-MIB in the solution to be measured.
As described above, in the surface acoustic wave sensor of this type, a variation in mass is detected as a variation in frequency. Specifically, the surface acoustic wave sensor is included in the oscillator circuit as a resonator, and a variation in mass is detected on the basis of a variation in frequency of the oscillator circuit.
Exemplary oscillator circuits using surface acoustic wave devices as resonators are disclosed in Patent Documents 2 and 3. The oscillator circuits disclosed in Patent Documents 2 and 3 are used in radio systems, such as voltage controlled oscillator circuits or remote controllers. FIG. 12 is an exemplary circuit diagram of an oscillator circuit of this type.
An oscillator circuit 121 uses a surface acoustic wave device 122 as a resonator. In the oscillator circuit 121, the surface acoustic wave device 122 is a two-port surface acoustic wave resonator. A first port of the two-port surface acoustic wave resonator 122 is connected to the gate terminal of a field effect transistor (FET) 123. A second port of the two-port surface acoustic wave resonator 122 is connected to the drain terminal of the FET 123.
An oscillation output is provided through an output terminal 124 connected to the gate terminal of the FET 123.
The oscillator circuit 121 in FIG. 12 uses the surface acoustic wave device as a resonator, as in Patent Documents 2 and 3. However, such oscillator circuits are used in the radio systems, such as the voltage controlled oscillator circuits or the remote controllers, and are not used for detection of a target substance.
In addition, in the oscillator circuit 121 in FIG. 12, a several-V DC voltage corresponding to a bias voltage of the FET 123 is normally applied to the interdigital electrode portion of the surface acoustic wave device 122 connected to the FET 123.
When the surface acoustic wave sensor, such as the one disclosed in Japanese Unexamined Patent Application Publication No. 10-90270, is used in the oscillator circuit 121 in FIG. 12, the following problems are caused. The surface acoustic wave sensor is normally dipped in a liquid, such as body fluid or blood, in order to detect a target substance with the surface acoustic wave sensor. Accordingly, part of the liquid is adhered to the surface of the surface acoustic wave sensor. In addition, when the surface acoustic wave sensor is used, instead of the surface acoustic wave device 122 in the oscillator circuit 121 in FIG. 12, a DC bias voltage is applied to the interdigital electrode portion of the surface acoustic wave sensor, as described above. As a result, with the DC bias voltage being applied, dielectric breakdown is caused through the liquid adhered to the surface of the surface acoustic wave sensor to cause the electrode in the interdigital electrode portion to be separated.
In other words, since the use of the oscillator circuit 121 or the like with the surface acoustic wave sensor to be dipped in a liquid is not assumed, the surface acoustic wave sensor used in the oscillator circuit 121 is not proposed.    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2-60211    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2-164121